Bumpless-transfer control system

ABSTRACT

A pneumatic controller has means for effecting bumpless-transfer between an automatic control mode of operation and a manual control mode; in the automatic mode, a pneumatic controller operates the process in a conventional manner, with the output signal to the process also being supplied to a follower device at an auto-manual station, positioning a mechanism continuously in accordance with the output signal to the process; upon transfer to the manual mode, the follower mechanism is placed into actuation by a manual set point, with the output signal to the process thereupon being provided from the manual-auto station; in addition, the output to the process is applied as a set point signal to a secondary pneumatic controller which is connected in a configuration with the primary pneumatic process controller such as will maintain the output bellows of the primary controller at the same pressure as the output signal to the process; to do this, the reset bellows of the primary process controller is supplied with a signal which will balance large deviations between measurement and set point supplied to the primary process controller; upon transfer from manual to automatic, the output of the primary process controller, being the appropriate pressure, is connected to provide the output signal to the process, and the secondary controller disconnected; deviation in output to the process from that demanded by the differential between measurement and set point is integrated in the normal pneumatic controller manner, according to the reset setting.

United States Patent 1 Aug. 1,1972

Beardsley et al.

[54] BUMPLESS-TRANSFER CONTROL SYSTEM [72] Inventors: Earl A. Beardsley;Hoel L. Bowditch; Robert C.Prescott,allof Foxboro, Mass.

[73] Assignees: The Foxboro Company, Foxboro,

Mass.

[63] Continuation of Ser. No. 772,593, Nov. 1,

1968, abandoned.

[52] U.S. Cl .137/86 51 Int.Cl. .I,.'.Fi5b 5/06,G05d16 /00 [58] FieldofSearch 137/86, 82, 85, 86 MA [56] References Cited UNITED STATESPATENTS 2,747,595 5/1956 Dickey ..l37/82 Primary Examiner-Alan CohanAttorney-Bryan, Parmelee, Johnson & Bollinger ABSTRACT A pneumaticcontroller has means for effecting bumpless-transfer between anautomatic control mode of operation and a manual control mode; in theautomatic mode, a pneumatic controller operates the process in aconventional manner, with the output signal to the process also beingsupplied to a follower device at an auto-manual station, positioning amechanism continuously in accordance with the output signal to theprocess; upon transfer to the manual mode, the follower mechanism isplaced into actuation by a manual set point, with the output signal tothe process thereupon being provided from the manualauto station; inaddition, the output to the process is applied as a set point signal toa secondary pneumatic controller which is connected in a configurationwith the primary pneumatic process controller such as will maintain theoutput bellows of the primary controller at the same pressure as theoutput signal to the process; to do this, the reset bellows of theprimary process controller is supplied with a signal which will balancelarge deviations between measurement and set point supplied to theprimary process controller; upon transfer from manual to. automatic, theoutput of the primary process controller, being the appropriatepressure, is connected to provide the output signal to the process, andthe secondary controller disconnected; deviation in output to theprocess from that demanded by the difi'erential between measurement andset point is integrated in the normal pneumatic controller manner,according to the reset setting.

15 Claims, 3 Drawing Figures PATENTEDAUG 1 I972 SHEEI 1 IF 2 mm n w 5%;1% l\ ml L M "+3 L05 Di @Nl film. mm Ad mml Q Q v m\ & t mm 2 0? mm QMIMxw m um 5925 S O @N Q EVE 5 mm M/QW ATTORNEY PATENTED AUE 1 I972 SHEET 2OF 2 I J{:31 113g MAN FIGQ MAN 25 FIGB BUMPLESS-TRANSFER CONTROL SYSTEMThis application is a continuation of Ser. No. 772,593, filed Nov. l,1968, now abandoned.

This invention relates to controllers having an automatic bumplesstransfer mechanism, and more specifically to a pneumatic controllersystem employing a secondary controller for maintaining the output of aprimary controller at the output pressure to the process during manualoperation, and employing a device for continuously following outputpressure to the process from the primary controller during automaticcontrol conditions.

The invention herein will be understood from the disclosure herewith inconjunction with the Figures in which:

F IG. 1 is a schematic type of diagram of a pneumatic bumpless transfersystem employing a primary process controller, a secondary controller,and a manual-automatic station;

FIG. 2 is an abbreviated representation of the relationship between theprimary process controller and the secondary process controller in theautomatic mode of operation;

FIG. 3 is an abbreviated representation of the relationship between theprimary process controller and the secondary process controller duringmanual operatron.

Referring to FIG. 1, a primary process controller 1 1 has a measurementsignal 12 supplied to measurement bellows l3 and a set point pressure 14supplied to set point bellows l therewith. Measurement bellows l3 andset point bellows 15 are opposing bellows on a force bar 16 pivoted atpoint 17, with another set of bellows 18 and 19 being opposed on theother side of pivot 17. Bellows l8 performs the integrating functionduring normal process control and bellows 19 is the output bellows,being connected through switching section 35C to supply a signal 20 tothe process during automatic operation. The switching configurationshown in FIG. 1 represents the manual mode of operation. A nozzle 21 anda relay 22 are operated in conjunction with force bar 16 and the outputbellows 19 in the automatic mode to produce a pressure in bellows 19such as will balance the various components of controller 1 1.

In the manual mode of operation, as is represented by the switching inFIG. 1, a secondary controller 23 is interconnected with the outputsignal 24 of follower 25 and bellows 18 and 19 of controller 11. Outputsignal 24, which is supplied to process output 20 in the manual mode, isalso supplied to bellows 26 of controller 23 as a set point type ofsignal. Controller 23 is activated in the manual mode by an enablingswitching signal from automatic-manual switch 27, wherein a supplysource 28 is connected through section 27A and restrictor 29 to bellows30 of controller 23. Bellows 30 in conjunction with noule 31 performs aproportioning function and the pressure from bellows 30 is transferredthrough restrictor 32 to bellows 33, performing an integrating function.Bellows 34 of controller 23, opposing set point bellows 26, performs themeasurement function, with the pressure from bellows 19 of primarycontroller 11 being connected directly into measurement bellows 34 ofsecondary controller 23. The function of controller 23 during the manualmode is to maintain the pressure in bellows 18 at such a value as willproduce a resultant pressure in bellows 19 of controller 11 and thus thepressure in bellows 34 of controller 23 at the set point pressure inbellows 26. That is, the manual process pressure from manual station 25is made to appear in the output bellows 19 of controller 11 duringmanual operation by means of controller 23. This is effected by aswitched connection 35A which in manual connects the pressure in bellows33 of secondary controller 23 re reset bellows 18 of primary controller11. During the manual operation the reset capacity 45 is disconnectedfrom relay 22 by switching section 35B, while the output from bellows 19is disconnected from process output 20 by switching section 35C.Switching is enabled through section 27A of manual-automatic switchwhich sends a pressure in the manual condition to sections 35A, B, and Csuch as connects 35A and disconnects 35B and C.

The manual station 25 contains a mechanism more fully described incopending US. Pat. application No. 772,597. Briefly, during manualoperation a mechanism 36 is positioned by manual set point knob 37operating through a linkage 38 to control a nozzle 39 connected to arelay 40 which through switching section 27B supplies a rebalancingpressure to bellows 41, as well as a pressure directly to process output20. The bellows 41 pressure, and thus the process output pressure 20 isindicated by pointer 42.

In the automatic mode of operation sections 27A and 27B of transferswitch 27 are opened thus interrupting the relay 40 output, andde-activating secondary controller 23, and placing switch sections 35A,35B and 35C in their automatic positions; section 35A is disconnectedand sectors 35B and 35C are connected. At this time, during theautomatic phase, the process output 20 is derived from bellows 19 ofprimary controller 11 through connected switch section 35C; the signalsupplied to process output 20 is connected into bellows 41 whichmechanically positions sector gear 36 through linkage 38 to a positionappropriate to the process output pressure. Also,during the automaticmode, manual knob 37 is detached from the sector gear 36, so thatbellows 41 is free to operate to position sector gear 36 in relation tothe output pressure at 20.

Bumpless transfer from manual to automatic occurs inasmuch as theconnection of bellows 19 to control the process through switch section35C finds the pressure in bellows 19 the same as the signal alreadyfurnished to output 20 from the manual station 25. Interconnections 43and 44 connect the switching signal and the output 20 signalrespectively between the automanual components 25 and 27 and the primaryand secondary controllers 11 and 23. The configurations thereby providedallows the removal of the component group comprising follower 25, relay40 and switch 27 as a sub-unit from the system, without influencing theautomatic operation of the remainder of the system. This is because theswitching signal supplied through line 43 is passive or atmospheric inthe automatic mode, and because line 44 incorporates sufficientrestriction so that during removal of station 25 bellows 19 ofcontroller 11 in conjunction with relay 22 may supply a sufficient flowof output 20 signal.

FIG. 2 represents the relationship between primary controller 11 andsecondary controller 23 in the automatic mode of operation. Very simply,primary conpressure from section 27A is atmospheric, so that bellows 30and 33 of secondary controller 23 are de-activated, having atmosphericaccess. Bellows 26 and 34, the set point and measurement bellowsrespectively of secondary controller 23, are both connected to output sothat they haveappropriate pressures therein should controller 23 beactivated by transfer to manual mode. In addition; output 20 supplies apressure to manual station 25, which is operated during automatic in afollower mode, in which a member is mechanically positioned according totheoutput pressure 20. This permits the manual station 25 to be switchedto manual operation with the manual knob connected through a gear to abellows which already has the output pressure last remembered fromautomatic operation therein which output may thereupon be changedmanually.

FIG. 3 shows the relationship between primary controller:ll andsecondary controller 23 inthe manual mode, in which the output of manualstation 25 is supplied both to process output 20 and to set pointbellows 26 of secondary controller 23. Secondary controller 23 has itsmeasurement bellows 34 connected to output bellows 19 of controller 11,and has reset bellows 33 connected to bellows 18 of primary controller11. Controller 11 has its reset tank 45 disconnected in the manual modeso that the output from bellows 30 of secondary controller 23 viarestrictor 32 producesan appropriate pressure in bellows 18 of primarycontroller 11 such that the output from'bellows 19 of primary controller11 is maintained at the pressure provided to output 20.

Desireable features of this system inhere in the arrangement whereby themanual group 25, 40 and 27 may be removed without interrupting automaticcontrol, and whereby theautomatic group 11, 23 may be removed withoutinterrupting manual control.

It is obvious that controller 23 may conveniently be of any conventionaltype, such as reset, derivative, proportional, or combinations thereof.Similarly, this system can be employed with electronic controllers byadaptation obvious to those skilled in the art.

While there has been shown what is considered to be a preferredembodiment of the invention, it will be manifest that many changes andmodifications may be made therein without departing from the essentialspirit of the invention.

We claim:

1. In a known process control system of the type having a controllerincluding:

input means for a measurement signal;

input means for a set point signal;

comparison means coupled to said input means to produce a forceresponsive to the difference between the measurement and set pointsignals;

a balanceable element to which said difference force is applied; 3

means to detect any imbalance of forces applied to said balanceableelement;

negative feedback means responsive tosaid detection means for producinga rebalancing force applied to said balanceable element;

output means for producing an automatic output signal corresponding tosaid rebalancing force;

a manual signal device for producing an adjustable manual output signal;i

a final control element cooperating with the process to be controlled;and 7 transfer switch means operable into automatic or manual state,said switch means serving in automatic state to direct said automaticoutput signal:

to said final control element and serving in manual state to direct saidmanual signal to said final control element; that improvement in suchapparatus comprising: second detection means operable when said switchmeans is in manual state to detect any difference between said automaticoutput signal and said manual output signal; controller conditioningmeans responsive to said second detection means when said switch meansis in manualstate and operable to produce a conditioning force appliedto said balanceable element corresponding to any difference between saidantomatic and manual signals; said first comparison means serving inmanual state to continue to apply to said balanceable element said forcecorresponding to the difference between said measurement and set pointsignals, just as in automatic state; I

said negative feedback means functioning in manual state to maintainsaid' balanceable element in balance with said automatic output signalequal to said manual'signal, whereby uponswitchback from manual toautomatic state the signal directed to said final control elementremains constant so as to prevent disturbing the process.

2. Apparatus as claimed in .claim 1, wherein the controller is of thetype including reset means to apply a time-variant force to said movableelement to provide reset action in said automatic output signal;

said conditioning means including means operable in manual condition toactivate said reset means to apply said conditioning force to saidmovable element, whereby said reset means is maintained energized at alevel commensurate with the development of an automatic output signalequal to the manual output signal then being applied to the controlelement.

3. Apparatus as claimed in claim 2, wherein said con ditioning force isapplied by said reset means to said first movable element in oppositionto said rebalancing force.

4. In a known process control system of the type having a controllerwhich includes:

input means for a measurement signal; input means for a set pointsignal; comparison means coupled to said input means to produce a forceresponsive to the difference between the measurement and set pointsignals; a balanceable element to which said difference force isapplied;

means to detect any imbalance of forces applied to said balanceableelement;

negative feedback means responsive to said detection means for producinga rebalancing force applied to said balanceable element;

output means for producing an automatic output signal corresponding tosaid rebalancing force;

a manual signal device for producing an adjustable manual output signal;

a final control element cooperating with the process to be controlled;and

transfer switch means operable into automatic or manual state, saidswitch means serving in automatic state to direct said automatic outputsignal to said final control element and serving in manual state todirect said manual signal to said final control element;

that improvement in such apparatus comprising:

second comparison means to produce a second force responsive to thedifference between said automatic output signal and said manual outputsignal;

a second balanceable element to which said second difference force isapplied;

second means to detect any imbalance of forces ap plied to said secondbalanceable element;

second negative feedback means responsive to said second detection meansfor producing a rebalancing force applied to said second balanceableelement; and

second output means operative when said transfer switch means is inmanual state for producing a conditioning force applied to said firstbalanceable element corresponding to said second negative feedbackforce, whereby when the system is in manual state the automatic outputsignal is maintained equal to the manual output signal so thatswitchback from manual to automatic state does not affect the signalsupplied to the final control element.

5. In a known pneumatic process control system of the type having acontroller which includes:

first pressure-responsive means for producing a first forcecorresponding to a pneumatic measurement signal;

second pressure-responsive means for producing a second forcecorresponding to a pneumatic set point signal;

a balanceable element adapted to receive said first and second forces inopposition;

negative feedback means for producing a rebalancing force applied tosaid first balanceable element responsive to said movement of saidelement;

output means for producing a pneumatic automatic output signalcorresponding to said first rebalancing force;

manually-adjustable means for producing a pneumatic manual outputsignal;

a final control element adapted to receive a pneuthat improvement insuch a known process control system which additionally comprises atransfer conditioning unit including:

means operable when said transfer switch means is in manual state toproduce a pneumatic conditioning signal corresponding to the differencebetween said automatic output signal and said manual output signal; and

third pressure-responsive means responsive to said conditioning signalfor applying a third force to said balanceable element related to saiddifference signal and in a direction such that during manual operationsaid balanceable element is continuously rebalanced to maintain saidautomatic output signal equal to said manual output signal.

6. Apparatus as claimed in claim 5, wherein said thirdpressure-responsive means comprises pneumatically operable reset means.

7. Apparatus as claimed in claim 5, wherein said transfer unit includes:

fourth pressure-responsive means operable to produce a fourth forcerelated to said automatic output signal;

fifth pressureresponsive means operable to produce a fifth force relatedto said manual output signal;

a second movable element adapted to receive said fourth and fifth forcesin opposition; and

means to detect movement of said second element and to produce saiddifference signal in correspondence therewith.

8. Apparatus as claimed in claim 7, wherein said known controllerincludes pneumatically operated reset means arranged to apply a force tosaid first balanceable element to develop reset action in said automaticoutput signal;

said transfer switch means serving in manual state to direct saidpneumatic difference signal to said reset means to cause said resetmeans to apply a force to said first movable element corresponding tosaid difference signal.

9. In a controller of the type having a balanceable control elementmovable in response to a deviation between measurement and set pointvalues, said movable element being co-operatively associated withsensing means responsive to the position of said element for producingan automatic output signal corresponding to the relative positioningthereof; there being adjustable means for producing a manual outputsignal and transfer switch means operable into automatic or manual statefor selectively directing either said automatic signal or said manualoutput signal to a regulating element such as a process valve;

that improvement in such a controller for permitting simplifiedoperation in transferring between automatic and manual state, comprisingthe combination of:

first conditioning means under the control of said transfer switchmeans; said first conditioning means including means operable, when saidtransfer switch means is in manual state, to apply to said balanceableelement a conditioning force responsive to any difference between saidautomatic and manual signals and serving to reposition said movableelement with respect to said sensing means so as to maintain saidautomatic and manual signals equal, in preparation for smooth switchbackto automatic state; and

second conditioning means operable when said transfer switch means is inautomatic state; said second conditioning means including tracking meansresponsive to said automatic output signal andoperable to continuouslycontrol the position of a signal-adjusting device in correspondence withthe magnitude of said automatic output signal, said second conditioningmeans further including means operable by said signal adjusting devicefor setting the value of said manual output signal equal to saidautomatic output signal at the time of switchover from automatic tomanual state, thereby to provide a smooth transfer to manual state.

10. A pneumatic controller comprising the combination of:

a balanceable control element movable in response to a deviation betweenmeasurement and set point signals, said movable element beingcooperatively associated with sensing means responsive to the positionof said element producing an automatic output signal corresponding tothe positioning thereof;

reset means coupled to said automatic output signal and arranged toapply to said movable elementa time-variant force to adjust thepositioning thereof to develop reset action in said automatic outputsignal;

the controller further including adjustable means for producing a manualoutput'signal and transfer switch means operable into automatic ormanual state for selectively directing either said automatic outputsignal or said manual output signal to a regulating element such as aprocess valve;

comparison means under the control'of said transfer switchmeans andoperable, when said transfer switch means is in manual state, to directto said reset means a continuous conditioning signal responsive to anydifference between said automatic and manual signals, said reset meansserving to reposition said movable element with respect to said sensingmeans. as required to maintain said automatic and manual signals equal,in preparation for smooth Switchback to automatic state;

said comparison means including a signal-producing device having meansoperable throughout the range of signal comparisons, including the crossover sign-change point, to produce a continuouslyvariable conditioningsignal free of dead-space effects and corresponding at all values withinsaid range to the difference between said automatic and manual signals,whereby to assure that the reset means receives at all times aconditioning signal of proper value to maintain equality between saidautomatic and manual signals.

11. Apparatus as claimed-in claim 10, wherein said transfer switch meansincludes means operable in manual state to interrupt the connectionbetween said reset means and said automatic output signal.

12. Apparatus as claimed in claim 10, including pneumatic restrictor andcapacity-tank means in the coupling between said comparison means andsaid reset means.

13. Apparatus as claimed in claim 10, wherein said comparison meanscomprises a force-balance member having differencing means for applyingthereto a force corresponding to the difference between said automaticand manual signals;

a pneumatic nozzle adjacent said force-balance member; feedback meansresponsive to the output of said nozzle to apply a rebalance force tosaid member to maintain the forces thereon in balance; and

means coupled to said feedback means for producing said conditioningsignal. 14. Apparatus as claimed in claim 15, wherein said differencingmeans comprises first and second pneumatic devices for applyingoppositely-directed forces toarranged to apply oppositely-directedforces to said first member corresponding respectively to measurementand set point values; t

a first pneumatic nozzle mounted adjacent said first member to producean output signal responsive to the positioning thereof;

negative feedback means connected to the signal produced by said nozzlecomprising a second set of pneumatic pressure-responsivedevices'developing oppositely-directed forces on said force-balancemember, and reset means coupled to said second set of devices to developreset action in the output signal from said nozzle;

adjustable means for producing a manual output signal and transferswitch means operable into automatic or manual state for selectivelydirecting either said automatic output signal or said manual outputsignal to a process regulating device;

a second force-balance member;

a third set of pressure-responsive devices arranged to applyoppositely-directed forces to said second a mem r;

means operable with said transfer switch means in manual state fordirecting said automatic and manual output signals to said third set ofpressureresponsive devices to apply to said second member a net forcecorresponding to the difference between said automatic and manual outputsignals;

a second pneumatic nonle adjacent said second member to produce acomparison signal responv sive to any unbalance of said second member; afourth set of pressure-responsive devices coupled to the signal of Saidsecond nozzle for applying oppositely directed forces to said secondmember to v said manual signal.

Patent No. 3,680,580 Dated August 1, 1972 Earl A. Beardsley et a1.

Inventor(s) It is certified that error appeers in the above-identifiedpatent and that said Letters Patent are hereby corrected as shownvbelow:

C01. 8, line l3, "15'' should read l3 Signed and sealed this 15th day ofMay 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. 7 ROBERT GOTTSCHAL K v Attesting OfficerCommissioner of Patents FORM PC4050 uscoMM-oc 60376-P69 US. GOVERNMENTPRINTING OFFICE: 199 0-365-334,

1. In a known process control system of the type having a controllerincluding: input means for a measurement signal; input means for a setpoint signal; comparison means coupled to said input means to produce aforce responsive to the difference between the measurement and set pointsignals; a balanceable element to which said difference force isapplied; means to detect any imbalance of forces applied to saidbalanceable element; negative feedback means responsive to saiddetection means for producing a rebalancing force applied to saidbalanceable elEment; output means for producing an automatic outputsignal corresponding to said rebalancing force; a manual signal devicefor producing an adjustable manual output signal; a final controlelement cooperating with the process to be controlled; and transferswitch means operable into automatic or manual state, said switch meansserving in automatic state to direct said automatic output signal tosaid final control element and serving in manual state to direct saidmanual signal to said final control element; that improvement in suchapparatus comprising: second detection means operable when said switchmeans is in manual state to detect any difference between said automaticoutput signal and said manual output signal; controller conditioningmeans responsive to said second detection means when said switch meansis in manual state and operable to produce a conditioning force appliedto said balanceable element corresponding to any difference between saidautomatic and manual signals; said first comparison means serving inmanual state to continue to apply to said balanceable element said forcecorresponding to the difference between said measurement and set pointsignals, just as in automatic state; said negative feedback meansfunctioning in manual state to maintain said balanceable element inbalance with said automatic output signal equal to said manual signal,whereby upon switchback from manual to automatic state the signaldirected to said final control element remains constant so as to preventdisturbing the process.
 2. Apparatus as claimed in claim 1, wherein thecontroller is of the type including reset means to apply a time-variantforce to said movable element to provide reset action in said automaticoutput signal; said conditioning means including means operable inmanual condition to activate said reset means to apply said conditioningforce to said movable element, whereby said reset means is maintainedenergized at a level commensurate with the development of an automaticoutput signal equal to the manual output signal then being applied tothe control element.
 3. Apparatus as claimed in claim 2, wherein saidconditioning force is applied by said reset means to said first movableelement in opposition to said rebalancing force.
 4. In a known processcontrol system of the type having a controller which includes: inputmeans for a measurement signal; input means for a set point signal;comparison means coupled to said input means to produce a forceresponsive to the difference between the measurement and set pointsignals; a balanceable element to which said difference force isapplied; means to detect any imbalance of forces applied to saidbalanceable element; negative feedback means responsive to saiddetection means for producing a rebalancing force applied to saidbalanceable element; output means for producing an automatic outputsignal corresponding to said rebalancing force; a manual signal devicefor producing an adjustable manual output signal; a final controlelement cooperating with the process to be controlled; and transferswitch means operable into automatic or manual state, said switch meansserving in automatic state to direct said automatic output signal tosaid final control element and serving in manual state to direct saidmanual signal to said final control element; that improvement in suchapparatus comprising: second comparison means to produce a second forceresponsive to the difference between said automatic output signal andsaid manual output signal; a second balanceable element to which saidsecond difference force is applied; second means to detect any imbalanceof forces applied to said second balanceable element; second negativefeedback means responsive to said second detection means for producing arebalancing force applied to said second balanceable element; and secondoutput means operativE when said transfer switch means is in manualstate for producing a conditioning force applied to said firstbalanceable element corresponding to said second negative feedbackforce, whereby when the system is in manual state the automatic outputsignal is maintained equal to the manual output signal so thatswitchback from manual to automatic state does not affect the signalsupplied to the final control element.
 5. In a known pneumatic processcontrol system of the type having a controller which includes: firstpressure-responsive means for producing a first force corresponding to apneumatic measurement signal; second pressure-responsive means forproducing a second force corresponding to a pneumatic set point signal;a balanceable element adapted to receive said first and second forces inopposition; negative feedback means for producing a rebalancing forceapplied to said first balanceable element responsive to said movement ofsaid element; output means for producing a pneumatic automatic outputsignal corresponding to said first rebalancing force;manually-adjustable means for producing a pneumatic manual outputsignal; a final control element adapted to receive a pneumatic signaland to cooperate with the process to be controlled in order to vary themagnitude of a desired condition; transfer switch means operable intomanual or automatic state adapted to direct said automatic output signalto said final control element during said automatic state and to directsaid manual output signal to said final control element during saidmanual state; that improvement in such a known process control systemwhich additionally comprises a transfer conditioning unit including:means operable when said transfer switch means is in manual state toproduce a pneumatic conditioning signal corresponding to the differencebetween said automatic output signal and said manual output signal; andthird pressure-responsive means responsive to said conditioning signalfor applying a third force to said balanceable element related to saiddifference signal and in a direction such that during manual operationsaid balanceable element is continuously rebalanced to maintain saidautomatic output signal equal to said manual output signal.
 6. Apparatusas claimed in claim 5, wherein said third pressure-responsive meanscomprises pneumatically operable reset means.
 7. Apparatus as claimed inclaim 5, wherein said transfer unit includes: fourth pressure-responsivemeans operable to produce a fourth force related to said automaticoutput signal; fifth pressure-responsive means operable to produce afifth force related to said manual output signal; a second movableelement adapted to receive said fourth and fifth forces in opposition;and means to detect movement of said second element and to produce saiddifference signal in correspondence therewith.
 8. Apparatus as claimedin claim 7, wherein said known controller includes pneumaticallyoperated reset means arranged to apply a force to said first balanceableelement to develop reset action in said automatic output signal; saidtransfer switch means serving in manual state to direct said pneumaticdifference signal to said reset means to cause said reset means to applya force to said first movable element corresponding to said differencesignal.
 9. In a controller of the type having a balanceable controlelement movable in response to a deviation between measurement and setpoint values, said movable element being co-operatively associated withsensing means responsive to the position of said element for producingan automatic output signal corresponding to the relative positioningthereof; there being adjustable means for producing a manual outputsignal and transfer switch means operable into automatic or manual statefor selectively directing either said automatic signal or said manualoutput signal to a regulating element such as a process valve; thatimprovement in such a controller for permitting simplified operation intransferring between automatic and manual state, comprising thecombination of: first conditioning means under the control of saidtransfer switch means; said first conditioning means including meansoperable, when said transfer switch means is in manual state, to applyto said balanceable element a conditioning force responsive to anydifference between said automatic and manual signals and serving toreposition said movable element with respect to said sensing means so asto maintain said automatic and manual signals equal, in preparation forsmooth switchback to automatic state; and second conditioning meansoperable when said transfer switch means is in automatic state; saidsecond conditioning means including tracking means responsive to saidautomatic output signal and operable to continuously control theposition of a signal-adjusting device in correspondence with themagnitude of said automatic output signal, said second conditioningmeans further including means operable by said signal adjusting devicefor setting the value of said manual output signal equal to saidautomatic output signal at the time of switchover from automatic tomanual state, thereby to provide a smooth transfer to manual state. 10.A pneumatic controller comprising the combination of: a balanceablecontrol element movable in response to a deviation between measurementand set point signals, said movable element being cooperativelyassociated with sensing means responsive to the position of said elementproducing an automatic output signal corresponding to the positioningthereof; reset means coupled to said automatic output signal andarranged to apply to said movable element a time-variant force to adjustthe positioning thereof to develop reset action in said automatic outputsignal; the controller further including adjustable means for producinga manual output signal and transfer switch means operable into automaticor manual state for selectively directing either said automatic outputsignal or said manual output signal to a regulating element such as aprocess valve; comparison means under the control of said transferswitch means and operable, when said transfer switch means is in manualstate, to direct to said reset means a continuous conditioning signalresponsive to any difference between said automatic and manual signals,said reset means serving to reposition said movable element with respectto said sensing means as required to maintain said automatic and manualsignals equal, in preparation for smooth switchback to automatic state;said comparison means including a signal-producing device having meansoperable throughout the range of signal comparisons, including thecross-over sign-change point, to produce a continuously-variableconditioning signal free of dead-space effects and corresponding at allvalues within said range to the difference between said automatic andmanual signals, whereby to assure that the reset means receives at alltimes a conditioning signal of proper value to maintain equality betweensaid automatic and manual signals.
 11. Apparatus as claimed in claim 10,wherein said transfer switch means includes means operable in manualstate to interrupt the connection between said reset means and saidautomatic output signal.
 12. Apparatus as claimed in claim 10, includingpneumatic restrictor and capacity-tank means in the coupling betweensaid comparison means and said reset means.
 13. Apparatus as claimed inclaim 10, wherein said comparison means comprises a force-balance memberhaving differencing means for applying thereto a force corresponding tothe difference between said automatic and manual signals; a pneumaticnozzle adjacent said force-balance member; feedback means responsive tothe output of said nozzle to apply a rebalance force to said member tomaintain the forces thereon in balance; and means coupled tO saidfeedback means for producing said conditioning signal.
 14. Apparatus asclaimed in claim 15, wherein said differencing means comprises first andsecond pneumatic devices for applying oppositely-directed forces to saidmember corresponding to said automatic and output signals respectively;said feedback means comprising third and fourth pneumatic devices forapplying oppositely-directed forces to said member; means coupling asignal from said nozzle to both said third and fourth devices; andrestrictor means in the connection between said nozzle and said fourthdevice.
 15. A pneumatic controller comprising the combination of: afirst force-balance member; a first set of pneumatic pressure-responsivedevices arranged to apply oppositely-directed forces to said firstmember corresponding respectively to measurement and set point values; afirst pneumatic nozzle mounted adjacent said first member to produce anoutput signal responsive to the positioning thereof; negative feedbackmeans connected to the signal produced by said nozzle comprising asecond set of pneumatic pressure-responsive devices developingoppositely-directed forces on said force-balance member, and reset meanscoupled to said second set of devices to develop reset action in theoutput signal from said nozzle; adjustable means for producing a manualoutput signal and transfer switch means operable into automatic ormanual state for selectively directing either said automatic outputsignal or said manual output signal to a process regulating device; asecond force-balance member; a third set of pressure-responsive devicesarranged to apply oppositely-directed forces to said second member;means operable with said transfer switch means in manual state fordirecting said automatic and manual output signals to said third set ofpressure-responsive devices to apply to said second member a net forcecorresponding to the difference between said automatic and manual outputsignals; a second pneumatic nozzle adjacent said second member toproduce a comparison signal responsive to any unbalance of said secondmember; a fourth set of pressure-responsive devices coupled to thesignal of said second nozzle for applying oppositely directed forces tosaid second member to maintain said second nozzle in balance; and meansoperable when in manual state for coupling the output signal of saidsecond nozzle to said second set of pressure-responsive devices, toadjust said automatic output signal to keep it equal to said manualsignal.